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Why 1st and 2nd class but not fast or slow airplanes?
Why do airlines separate passengers into 1st and 2nd class but do not offer fast or slow connections? Isn't the biggest cost the kerosene? And isn't the cost directly proportional to speed?

There are slow connections which cost less, which is accomplished by having long transit hours.

Private jets are an example of what you are asking about.  Some can travel near mach.  But they are MUCH more expensive than a commercial flight.

isn't the cost directly proportional to speed?

No.

Most fuel can be saved by flying higher because the lower air density reduces parasite drag. Provided the trip is long enough for the saving at altitude to offset the cost of getting there.
Aircraft drag has two components. The parasite drag which increases with speed and the induced drag which decreases. So they have a point of minimum drag and for jet airliners it's typically somewhere between 200 and 250 knots indicated. Flying any slower would increase the cost. At the altitude where jets normally fly, the true speed is significantly higher than indicated speed.
For jet engines, the fuel burned is roughly proportional to thrust they generate and at higher thrust lower per unit of thrust. Since power is thrust times speed, this translate to much better efficiency at higher speed. So a jet wants to fly slightly faster than their lowest drag point.
Even if the trip cost is higher, higher speed allows you to make more trips, so the fixed costs get split over more passengers. So it makes sense to fly slightly faster than the lowest fuel burn.

Generally airliners are operated at the point of lowest cost. For long trips where it pays off to climb high, jets are most efficient. For short trips, turboprops are often used that can't climb that high, but are more efficient at lower altitudes and speeds.

Can you cite a reference for the 250KIAS figure?

I changed 'reduces drag' to 'reduces friction,' and someone changed it back. There are many different causes of drag, and friction is one of them, the one which is alleviated by lower air density.

@rbp: Lower air density reduces more than friction, but indeed not all drag. The most precise without complicating the discussion is probably using 'parasite drag' or 'form drag'. And the end effect is not that the aircraft flies with less total drag, but that it flies faster with given drag.

@rbp the 250 is the general imposed limit below 10,000 ft in the US, maybe that's where the figure came from.

@BurhanKhalid: No, definitely didn't come from that. Probably a little too high too; I recall reading A320 has Vy of 220 KIAS, but not sure where. The actual value varies between aircraft and for given aircraft with weight. And best glide distance speed, which is more relevant here, is higher than Vy (though I wrote minimum drag and that means Vy).

@BurhanKhalid   there is no reference for 250KIAS as L/Dmax because there isn't one.

@JanHudec Parasite drag is the right edit.

Concorde was built for speed. But it was costly too. People did not like it much. Reality was that they didn't want to get somewhere faster by paying more. People love to pay for luxury though. That is the reason different travel classes were introduced since late 1970s.
Cost of fuel and speed are NOT directly related. As you may know, Boeing 787 has increased fuel efficiency (claimed to be the most fuel-efficient), and still reaching higher speeds.

+1 for mentioning the Concorde, but it was well received by the comparatively few pax who rode on it thanks to the shorter flight time, lower cabin altitude, and good food. And hey, the view at 60000 feet is pretty good too!

@shortstheory yes, but they lambasted the narrow, cramped, seats, the lack of legroom, etc. etc. Effectively you were paying first class rates for cattle class seating with business class service.

If I recall correctly, as concorde was not invented in the US, it was not allowed to fly at full speed over the US, hence limiting it’s usefulness.   Also due to time difference, it only worked well for going from the UK to the US, as the other way a full size bed in the aircraft was a better option than having half a night’s sleep in a hotel.

@IanRingrose, the ban on over-land supersonic flight wasn't because Concorde was European and not American. It would have applied to all supersonic airliners, and is one of the major reasons the US SST was cancelled. Boeing took a huge hit when they cancelled the 2707.

I flew the Concorde to France, and loved it! The "narrow cabin and lack of legroom" was a myth cited by people who'd never flown to Europe in 2.5 hours

@rbp I cannot say about legroom but narrow cabin is not a myth. B737 has cabin width of 12'4", whereas Concorde is 9'5".

@Farhan, sure, but 737 (coach) is 3x3, while Concorde was 2x2. Per person, you'd have more room on the 'smaller' airplane.

If you couldn't sit in a Concorde back when it was in Service, just do it now. When I was in the "Concorde Café" in the Aviation Exhibition in Hermeskeil, Germany in June 2014, I found the seats to have plenty of room; by far more legroom and width than a Fokker 50 business seat (I didn't fly Business or First in any jet, though; perhaps these are better?).

Can the person who downvoted please explain it?

I didn't downvote, but it's an overstatement that "people did not like it much". The people I knew who flew it said they liked it. I've been aboard one, and it was a little tight but the seating was nice and I quite liked the whole plane's aesthetic. If I'd had the opportunity and it didn't seem extravagant, I would choose it. Also I would never pay for Business class or First class (though I would choose a nicer airline).

@egid First class on a B737 is usually 2x2. Also, considering the aisle still has to exist, 3x3 at 12'4" would end up being about the same per person width as 2x2 at 9'5", if not a bit more. While not horrible, that's not exactly spacious given what the tickets cost on Concorde. That's not to say I wouldn't love the opportunity to fly on one (especially if someone else was paying for it,) but I can see how people would complain about it being cramped (both horizontally and vertically) given the cost. I personally find CRJ900s to be cramped and Concorde was of a similar diameter.

@Dronz I know you didn't downvote :) I was referring to cost when I said that people didn't like it.

The optimal speed of the aircraft is set by the manufacturer.

isn't the cost directly proportional to speed?

No, any speed above or below the optimal one will lead to sub-optimal performance of the engine, increasing the costs.

Why airlines do not offer fast or slow connections?

The answer is partial speculation, but the costs do not increase linearly, and I think that the people that could afford such increase in costs (and would buy such a ticket) are not enough to fill standard commercial aircrafts. In fact I think they have their own personal and private business-jet (or something of this sort)

Good answer. And even with a fast private business jet, cruising at Mach 0.9 really doesn't get you there that much sooner than at Mach 0.75-0.85 (typical airliner range). To see a really significant travel time reduction you have to go supersonic (not currently an option unless you have access to military hardware :)

Private jets reduce your total travel time (door-to-door), not your airport-to-airport gate time. And they can use smaller airports closer to where you want to go, on your schedule. No checkin, baggage or security hassles either.

I think the majority of people flying first class (in US Domestic flights) have not even paid for their first class upgrade. Rather, I believe the majority of them have free upgrades granted due to their frequent flier status.

@paul And, in fact, higher classes can also reduce your door-to-door when you're on the same plane (you often don't have to wait in the same check in and security lines, at least in the US where there's the fast lines through security).

Providing a fast and a slow "class" for flight tickets would add a lot of overhead to airline operation in many ways  - way more than it could be worth it:
To have fast and slow flights, one could

Use two kinds of planes, fast and slow:

Requires big, slow planes and fast, smaller planes, assuming fewer seats are sold in the fast, expensive class. An airline that has only one size of planes now would require a broad range of types of planes.
Cannot handle flights that now use relatively small planes when the fraction of fast-class passengers is low, as for very few it get's just too ineffective to have a separate flight.

The potential solution of downgrading the high-price customers to slow flights does not sound useful.
Solving it by upgrading the slow-class passengers to a fast flight does not work in general as the fast planes needed otherwise are too small for the higher count of passengers (or would at least make planning even harder).

Use one kind of plane, used to fly fast or slow:

Needs about twice as many planes compared to now.
Makes seat allocation planning harder - need to estimate counts per speed class for planning, which may change depending on external events (like trade fairs at destination being consumer or business-targeted)
Will provide not much benefit to the passenger, as the difference between the fast and the slow flight speed is limited by the physics of the planes used

The slow speed would need to be the most fuel-effective speed, to support low prices. That is the speed of best L/D ratio - so the slow speed could not be much slower that today's standard speed, as that would cost extra fuel.
The fast speed would be limited by the highest speed of the plane allowing acceptable fuel use, and by the power that the engines of the airplane can provide (assuming it's save to fly at that speed).
Because the fast-class speed is faster than the slow-class speed, it's above the speed of best L/D ratio. For speeds in that range, the induced drag is dominated by the form drag, and the from drag increases with the square of the speed.
At some speed, looking at total fuel usage it does no longer make economic sense to go faster, and because of the square relation, I expect the possible speed difference to be "not large enough to make sense" to distinguish two completely different flight classes.

It could work, though, if we take large airships into account, and providing very slow, but very cheap flights, but still faster that water-based ships.

The problem with the airship is that JFK-LHR will need two days, which means you need six meals, four liters of beverages, a seat and a bed per passenger, which all adds up to the bill. And then, no-one has two days to spare these days. Either they go on a five-days-or-more cruise aboard Queen Mary (which costs way more than a first-class flight, by the way, but then, there won't be a swimming pool aboard your airship), or they just take a plane.

In thinking about it, the 1st, 2nd and 3rd class distinction originally arose in the time of watercraft travel and during those days, there were also significant distinctions in the speed/cost tradeoffs on various types of vessels. Hence the term, "slow boat to China."
The slowest and cheapest passage was to ride supercargo (literally above the cargo) on the slowest cargo vessels such as barges. The fastest and and sometimes most expensive expensive passages were on packet ships, ships built for raw speed and used to carry mail. They were small, cramped and took a pounding from the sea. People seldom took passage on them if they didn't have to. When you read a historical reference to someone important coming in a packet ship, it would be like flying in a VIP in the rear seat of fighter jet today.
The classes came from accommodations and might have come from the British practice of rating ships themselves, 1st, 2nd and 3rd class. They really only appear in the steamship era when purpose built passenger ships evolved. Ocean liners made a point of their speed, sometimes beating packet ships because of their long hulls, but since even a short trip was matter of weeks accommodations mattered.
So why didn't the air travel industry break down into slow and fast planes? Others have noted the scaling problems with speed vs cost but another major reason is the difficulty in accommodating aircraft flying at different speeds at the same airport.
The length and strength of runway, for example, has to be proportional to the weight and takeoff/landing speed of the aircraft that use it. A slow light plane on long,strong runway is a waste and the opposite impossible. Likewise, it's hard to safely coordinate planes all flying around at different speeds with different safety intervals. That's why we have the distinction between military, commercial and civil aviation airports, each supports a different array of aircraft.
Contrast that with ships in smaller vessels and larger ones skim right by each other (at least at lower harbor speeds.) Harbors used to be crammed with all sizes and speeds of vessels sailing safely by each other and that in turn meant there was an economic niche for both fast/expensive and slow/cheap passage rates.

To offer different rates for different speed of travel the airline would need to add different aircraft types to their fleet, which would increase their cost of doing business dramatically.
They'd need to have a slow type for the lower price, a medium speed type for the "regular" price, and a fast type for the "high speed rate".
If those even exist in the range/size bracket the airline is planning to introduce that service in, they'd now have to have multiple groups of crews, several more maintenance facilities with crews trained in maintenance of different aircraft types, a vastly increased amount of parking space for storing the aircraft in between missions, massively increased spare parts inventory, etc. etc. etc..
All that would mean the cost of running the scheme would rapidly overshadow the potentially higher income. And at the same time the customers would run away from the lower speed option to take the train of fly with a competitor who offers the same route on a faster aircraft using a more traditional service based pricing policy.

At the moment, planes travel at around mach .75 to .95; which is a tradeoff between speed and fuel consumption.
While it is possible to produce faster airplanes, no one does, because significantly faster (=supersonic) planes are not allowed everywhere; apart from that, they need much more fuel.
But to improve time, you don't have to consider using faster planes on some legs, you have to consider the whole trip.
Which may be:

90 minutes from manhattan to airport during rush hour
Check in, security check: 90 minutes
Six hours on a 747, 3 on a Concorde.
90 minutes waiting for the connecting flight (if you are lucky).
Three hours on a 737.
Half an hour by cab to your Oslo office.

Which makes your whole trip 14 hours on a normal plane, or 11 hours if you take Concorde, doubling(?) the price. I guess that heli transfers from and to the airports would be better investments than a seat on Concorde, if your company has helipads on the roof.
Overall trip times are why Boeing decided to not supersize their planes even more, and opt for more direct connections instead:

90 minutes from manhattan to airport during rush hour
Check in, security check: 90 minutes
7.5 hours on a 787
Half an hour by cab to your Oslo office.

And voilà, you are down to 11 hours, without the need for fancy supersonic planes.
By the way: While flying all-Economy is what some low-fare airlines do for a living; no one was ever successful in trying scheduled all-First flights. First is mostly combined with Flex. So, while First is a good thing on top, Economy is the bread-and-butter business which attracts the Flex-First bookers, because Economy is what allows the airline to do seven trips a day and still have a spare seat for the VIP on whichever connection he chooses.

There is a faster option - it's a case of looking at total travel time - door to door. In which case private jet is faster.

It goes when you want it to - no waiting for the 747 or A380 to go once a day - saving up to 12 hrs on journey
No checkin - minimal formalities (esp if you e.g. head of state) - saving 1hr or more?
It goes direct to the nearest airport the aircraft can access. No going via super hubs like Chicago or Heathrow. Saving 3+ hours.

Flying super fast between hubs doesn't save that much time compared to aircraft flying direct to where you want.

This is sort of fast -vs- slow in air travel....  I don't think that this is what the question was actually asking about though.

There is indeed a benefit in flying more slowly, but the aircraft needs to be designed that way to profit from slower speeds. You can see that wing sweep has been slightly reduced in the more modern airliners, but this is also due to better airfoils with higher Mach drag rise onset.
But some things will also cost more when you slow down: You have to pay the crew for more hours, and the aircraft cannot be utilized as often. To transport the same number of passengers requires more aircraft when they fly at a lower speed. The cost versus speed relationship is not linear, but it should have a minimum somewhere below today's operating points. Aircraft with a slower design cruise speed can be lighter and smaller for the same payload, but for now airlines don't have a choice of slow or fast jets; all manufacturers try to fly at least Mach 0.83.
This has a lot to do with marketing: The aircraft with the higher speed will show a shorter travel time for the same distance, so it will show up first in the booking systems of travel agents. Of course, now you will argue that most people book on the internet and will try to get the lowest cost, regardless of travel time. True. But this is not the sort of customer the airlines are after. Their profit comes from people in First and Business class, and these still book predominantly via travel agents. Therefore, Airbus and Boeing try to market their planes as the ones which come first on the Amadeus and Sabre screens.

You have many good points, but I think some things are not correct. The biggest benefit is flying higher and at the usual cruise altitude the jets actually fly at their best fuel economy speed.

Maintenance costs also go up if you put more time on the airframe/engines.

@Jan Hudec: The main benefit of flying higher comes from the lower temperatures. Once you climb above the tropopause, this effect goes away. What remains is less friction at the same speed, but also less dynamic pressure. And you need that to create enough lift. Most airliners cruise already at their maximum operating altitude (at the given mass), if possible. Less fuel consumption can only be gained from flying slower, and with an optimized airframe for that speed.

@PeterKämpf: Both lift and drag are functions of dynamic pressure and the benefit stems from the fact that the same dynamic pressure with minimal drag is at higher true speed. So the benefit does not go away above tropopause, it increases even faster. Limiting factors are the wave drag when approaching speed of sound (which by the way is lowest in tropopause and increases above as it increases with temperature) and amount of oxygen available for the engines to burn.

Oh, come on, Jan, you should know that the benefit only occurs if the plane is not flown at its design point. Flying higher means quickly growing engine demands which allow for nice short take-off runs and fast climbs, but make the airplane less efficient overall. And since temperature is roughly constant at the first few kilometers of the stratosphere, the speed of sound does not increase until you climb much higher. Current engines, runways and aerodynamics combine acceptable take-off and cruise performances at Mach 0.8 - 0.85 at FL 300 - 390. Improving this at the same Mach number is hard!


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Why don't airliners carry radar jamming technology or missile counter measures?
Why is it that commercial passenger aircraft that fly through dangerous areas don't carry some sort of electronic counter measure to defend themselves from missiles?  Between terrorism and the various war zones dotting the globe, it seems like it would be prudent for airliners to have them.  At least for aircraft that are flying in places that are possibly dangerous.
I've been thinking that it's probably a lot of expense and weight for something that isn't terribly likely to happen, but I wondered if there might be more to the answer or if, at very least, someone could add some details to my assumption.  A good cost/benefit analysis would be appreciated if anyone is familiar with the cost of operating such a system.

Odds are greater the threat will be internal (parts failure or on board physical terrorist) vs. external (ground to air, air to air). Though there have been more than one airliner shot down, more have been lost due to internal threats. Cost vs risk. The risk of being shot down, in compared to the massive number of miles flown is infinitesimal compared to a terrorist on board which can be compared to engine/elect/hyd failures.

@CGCampbell: Well, the solution to KAL007 and KAL902 was introduction of GPS as navigational error leading the aircraft into closed airspace was major part of such incidents.

Modern SAMs cannot be jammed easily. Besides radar they use a variety of thermal, acoustic, optical, wake detection sensors. A 75 m long airliner flying straight below Mach 1 would be hard to miss. Nothing could have saved MH17, once the decision was made to destroy it.

@stali If you can work that into the selected answer as a suggested edit, I think it would be a good idea.  You have a good point.  And at this point I'm really just trying to get everyone to put the strongest comments into the accepted answer.

You are assuming that all missiles use radar, which is not the case

@SteveKuo "missile counter measures" was meant to be the blanket statement that covered all other techs.  I was particularly curious about radar jammers when I wrote the question though, hence its in the title.

@stali: Nothing on the part of MH17, I assume you mean, given that it could easily have been saved by (say) the missile blowing up on the launcher.

El Al (and other Israeli) aircraft are equipped with infra-red countermeasures since fire at Arkira B757 in Mombasa in 2002. Infra-red counter-measures are relatively simple (it has a small radar to detect missiles and if one is detected, dispenses burning decoys), so it can be mounted on almost any aircraft. The system however poses additional fire risk (the flares have to burn very hot and the system has to be able to ignite them), so it is only used in areas where risk of terrorist attack is high. Switzerland (and possibly other countries) even prohibit it on their territory. Apparently laser-based system is in development to resolve the fire issue, but it will still only work against IR-guided missiles.
But for radar guided missiles like the Buk system probably involved in this incident there only seem to be three defences:

Hit the tracking radar with your own missile (like AGM-88 HARM) before they hit you. Fighter aircraft is obviously needed to carry it, though most fighter types could be used as they only need radar detector and suitable hardpoints.
Generate sufficiently intense microwave signal that it overdrives the detector in the missile head and effectively blinds it. This is heavy, expensive and requires power (not outside the range of common aircraft generators, but the increased fuel burn would probably be noticeable), so even most fighters don't have it. Instead the system (like AN/ALQ-99) is mounted on special-purpose aircraft like EF-111A Raven that gets included in attack group trying to penetrate enemy territory with heavy air defences.
Minimizing radar cross-section, ie. stealth. That requires completely different designs that are aerodynamically less efficient, because the funny shapes are necessary part of being stealth and that are not trackable by ATC surveillance radars.

Obviously the first is out of question for airliners - carrying any weapons or munitions of war on civil airliners is prohibited by Convention of International Civil Aviation article 35.
While the second would be technically possible (E-4B, an airborne command centre, and VC-25, the Air Force One, are modified Boeing 747s and both have counter-measures against both IR-guided and radar-guided missiles), those modifications would be a heavy, expensive and complex protection against something that has only happened a few times.
And the third would have too many disadvantages. It would hide the aircraft from ATC surveillance radars that have averted many more accidents than were caused by missiles and it would require new, less efficient designs that would be massively expensive. Just few month ago we wanted MH-370 to be seen by radars. We can't have both.
Staying away from the war zones is a safer option anyway and is normally possible. In this case likely explanation why MH-17 didn't is that the Ukrainian authorities were not aware that the rebels have this air defence system, assumed they only have a lighter-weight one (those are often usable to about 20000'; the military aircraft shot down on previous days were lower) and only closed the airspace up to FL320 rather than completely.

"Why is advanced weaponry obviously not an option for regular airliners?" Because the weapon will one day be accidentally fired, be stolen by terrorists, explode due to a system failure, explode during an otherwise minor accident, be fired by a resentful pilot, be fired at a harmless radar station that the pilots thought was targetting them, be fired at a friendly military radar that mistook the airliner for an intruder, etc. etc.

I'll add a couple more points about something like the HARM missile.  It's a race between two weapons which both operate at around Mach 3--killing the guidance radar before the missile gets you is hard enough for the guys whose job it is to hunt the SAMs, let alone for a jetliner.  Furthermore, it takes a lot of electronics and a skilled operator to figure out exactly what the target is and tell the HARM that.

@Articuno putting any weaponry on civilian airliners would mean that they are not civilian airliners anymore. Convention of International Civil Aviation article 35 prohibits any "munitions of war or implements of war", even carrying them as cargo.  If an airliner would be equipped with air-to-ground missiles, then it would require explicit permission and flight coordination from each and every country it flies over, as any other scheduled military flights over foreign space; and they'd have full rights to arbitrarily deny it, unlike civilian flights.

@JayCarr: Since ICAO didn't seem to protest against Israeli airlines installing flares in some of their aircraft these apparently are not considered weapons. USA and Switzerland do not want these aircraft on their territories, but it is due to increased fire risk (system that blinds the missile by bright infra-red laser instead seems to be in development to resolve that), not due to being weapons.

Hiding from ATC surveillance radars, already solved with squawk - even B-2's do that.  For a high-energy defensive weapon, you don't need to add weight/fuel burn of more generators if you can make it on the power intended for cabin/galley/nonessential loads. Shut em off - this is called "load shed" and jetliners can already do that.

@Harper, you mean showing up on them, right. The problem is that when you squawk, you are no longer stealth (unless you squawk with that DSS crypto weirdness, but ATC depending on anything secret is non-viable).

Well, you could have some fun with phasing and synthetic aperture and directionality... It's not like the location of ATC radars is a secret.  General concept in any case, when someone is shooting at you, stop squawking.

@Peteris: Even purely-defensive systems (like some sort of CIWS)?

@Sean The wording is "Each state shall determine by regulations what constitutes munitions of war or implements of war for the purposes of this Article", but in general implements of war includes pretty much all military gear. Spare parts for an Abrams tank would qualify. Ammo for CIWS would qualify. If you need to get a box containing CIWS from point A to B, you get a proper permit for carrying that cargo from all countries you overfly, instead of sticking it on a random civilian flight. This was an issue in e.g. supplying Afghanistan operation, as the flights had to take the long way around.

@Sean furthermore, CIWS is clearly not a purely-defensive system - it's generally based on a high-speed automatic cannon, it's a weapon even if you intend to use it defensively (just like a machine gun nest or a cannon in an immovable pillbox is a defensive position, but clearly a weapon). It's mostly safe to use in open sea, but firing it (for example) near an airport on approach or landing is likely to kill people.

Surface-to-air missile attacks on airliners at cruising altitude are vanishingly rare: there have only been three known attacks in the past half-century (MH-17, IR-655, and probably S7-1812).  Any protection system needs to take that into account -- any safety system needs to be justified in terms of cost, weight, and safety -- it needs to save more lives than it costs.

Direct jamming.  Suppose you stick an AN/ALQ-99 jamming pod on the airplane.  That's 6000 extra pounds, a good deal of drag, and a bunch of parts that can break off, catch fire, or otherwise be a hazard.
Active countermeasures.  Stick an AGM-88 HARM missile on the plane.  Now, your anti-radar missile isn't much faster than the radar-guided missile trying to shoot you down, so for reasonable safety, you need to launch as soon as you get painted by the tracking radar.  What do you suppose the odds are that a twitchy co-pilot acting as ECM/weapons officer is going to launch against an innocent control tower?  How many weather radars are you willing to destroy to save that one airplane every 20 years?  Are you sure that missile will never malfunction on the launching rack and destroy the airplane?
Stealth.  Hiding from radar-guided missiles means you're also hiding from traffic-control radar.  How often are crashes prevented by that radar?  I bet it's more than one every twenty years.

The benefit is roughly one less airplane crash every 20 years.  That means the cost, both in terms of money and in terms of risk, needs to be very, very low.

+1 for adding what should obvious Hiding from radar-guided missiles means you're also hiding from traffic-control radar.

Is it ironic that people were complaining about MH-370 not being seen by radar, and now MH-17 because it was?

@CGCampbell By very definition if I'm not mistaken.

I was wondering if the author could see about adding point 3 to the accepted answer as a suggested edit (since I think points 1 and 2 are more or less covered).  I'm trying to get all the best answer into once place...

+1 for "vanishingly rare": the extreme unlikeliness of such events seems to escape most people. Downing a civilian airplane is guaranteed to make international news, and airplanes are special in how far people are willing to go in suggesting counter-measures to unlikely events while also being very much against many effective counter-measures for rather likely road traffic deaths...

@Sean it matters because radar jamming technology and other electronic countermeasures won't protect you from a small shoulder-mounted missile fired from close range on final approach, like these incidents.

You answered your question already. Weight limitations, cost of installation and maintenance and likelihood of the system really being required or used - it is just not worth it for carriers.

It's a good assumption, but do you have any facts or figures on that?

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When a plane flies faster than the speed of sound, does the distance between plane and sound increase?
I had this discussion with my dad about a question he had asked. Basically he said:

If a plane flies faster than the speed of sound, will he be able to overtake his own sound and hear itself?

(let's leave behind the fact that he can run out of fuel or that the sound finishes before he hears it)
I disagreed because it seemed illogical to me, but we never actually found hard evidence to determine if the statement is true or false.
Can a plane actually do this?

If the plane is already flying faster than sound, then the sound is never "ahead" of the plane to be overtaken - it is behind you as soon as it is generated. You would still hear sound as transmitted through the airframe though. If you start below the speed of sound and accelerate through it, you do indeed "catch up" with the sound you've already generated, resulting in a sonic boom.

As you've probably gathered from the accepted answer, your dad has it backwards. When not moving the plane always hears itself ("hear" means the same as you can hear yourself talking). When it's faster than the speed of sound it can no longer hear itself because it left the sound behind it (minus whatever sound transmitted via the airframe).

@slebetman thanks for clearing that up but what do you mean with the ariframe?

Airframe = body, wing, tail etc. Basically the physical structure of the airplane. Of course the airplane is more than just the airframe. An airplane = airframe + control surfaces + electronics + engine etc.

@slebetman ahh i understand, it's the frame on which the aircraft's housing is build and the part that keeps the plane together

Your dad does have a fairly correct intuition about hearing sound twice though. But generally you can't hear it in the airplane itself. Or you can but by then it would be moments before you die and the aircraft disintegrating. Hearing sound produced by the airplane multiple times simultaneously is actually what the sonic boom is. A sonic boom is merely the normal sounds the airplane makes (engine rumble, the swish of flowing air etc.) arriving all at once (see picture from Fedrico's answer).

Sonic boom at source is very destructive. It causes control surface failure, engines to fail, violent vibrations etc. It's why fast jets have long noses and swept wings. To delay the pressure wave from touching the aircraft body (or at least important parts of the aircraft) as late as possible. That's also what practically limits airplane speeds. The faster you want to go (and survive) the longer nose and more wing sweep you need.

Here's a link to a handy demo video my friend made:  https://www.youtube.com/watch?v=TklvFQsBxRM  ;)

@MichaelHall wow that's actually really cool!

@BRHSM, FYI that was actually a joke video he made, it doesn't actually work like that.  The air package inside the cockpit is moving along at the same speed as the occupant so the sound waves would propagate uniformly from the source.  It may be useful to think of it in terms of ripples on a body of water.  Drop a rock and ripples are circular.  A slow moving boat will push some waves ahead of it.  But a speed boat will get there before the ripples (wake) it generates.  The sonic boom is just the sound equivalent to a  wake hitting the shore after a boat passes.

I think you are missing some physics knowledge, so let's start from here:

If a plane flies faster than the speed of sound

Sound is a compression wave that travels through air. Given a certain air temperature, the sound will travel at different speeds through the air.
If a plane flies faster than the speed of sound means that the compression waves it is generating will remain behind the aircraft:

image from here

will he be able to overtake his own sound and hear itself?

As you can see on the right picture, if the aircraft is travelling faster that the speed of sound, the compression waves it generated in the past will remain behind. If it would stop/slow down, the waves would then catch up with the plane and a person on board would be theoretically capable of hearing them.

does this also means that the dictance between plane and his sound will become greater over time?

@CoderGuy depends on the velocity, if it is not Mach 1, yes.

@Frederico mach 1 is the speed of sound right?

@CoderGuy yes, but labeled so that you do not have to know the temperature. (and it is FEderico, no extra r)

I don't know much about planes, but I know the "stopped" phase is really bad for planes. :o

@corsiKa I find it difficult to board a non-stopped plane.

Would I be correct in thinking that the speed of sound increases with pressure, and that in front of the plane there would be a region where the pressure was sufficient to elevate the speed of sound to that of the aircraft?  If a supersonic aircraft were to fly into an obstacle (e.g. a hailstone) I would expect that the hailstone would be hit by a shockwave slightly (somewhere between a microsecond and a millisecond) in advance of being hit by the aircraft itself.  Would that intuition be correct?

While it is a different question, I think this gets more interesting when you think of a jet flying in a racetrack pattern at a speed greater than Mach 1.  I doubt the airframe can handle turning fast enough, but if it could, I expect it would sound the same as a fly-by.

Your father asked:

If a plane flies faster than the speed of sound, will he be able to overtake his own sound and hear itself?

Of course he can!
He can catch up because he is faster. But that does not fully answer this brilliant question. We want to know what would practically happen:
But what does it sound like?
We want to catch up with our own sound, so it needs to get away from us first. That means we fly slower than sound in the beginning. For example, your father shouts

Can you hear me?

in the direction of travel.
Now, we accelerate our aircraft to be faster than sound. With that, we create a shockwave in front of the plane, which can be heard as the sonic boom. Practically, the whole situation is really loud
We hear no difference.
It does get louder, but not noticeable.
But in theory, we can think about what we would hear when it would be quiet, and we could use the air molecules in front of our shockwave as a microphone.
But if we could hear it anyway?
Then, we would actually hear "Can you hear me?", but in very deep voice, so deep that it is to deep to be heard by a human. The words were spoken in some seconds, but we reach the sound waves only slowly over some minutes.
We would hear a very deep voice...
And as we creep up to our sentence from behind, we catch the last syllable first, because that is what we said at the latest time, so it had less time to go away. The direction of the time in the sound is flipped.
Speaking the words backwards!

It's a very deep
¿em raeh uoy naC

@VolkerSiegel actually what you would hear is ¿em raeh uoy naC

@Jpe61 Oh yes, absolutely! I had thought about the direction for a second or so, and dismissed it. Too early, obviously. Thanks!

Here’s the answer you’re looking for and everyone has missed. The fact that inside the cockpit the reason why the pilot and whoever else is inside can in fact still hear things.
Inside the cockpit, sound travels as normal because it’s a controlled, pressurized environment where sound waves can still propagate. The pilot would be able to hear internal noises like engine hum, radio communications, and other cockpit sounds because those sounds are generated inside the plane and don’t have to pass through the outside air.
What the pilot won’t hear, though, is any sound coming from outside the plane (like the sonic boom it creates or other exterior noises) because the plane is moving faster than those sound waves. The sound waves generated by the plane get left behind it, so there’s no way for those external sounds to catch up and be heard in the cockpit.
Now of course if the cabin wasn’t pressurized, just a hypothetical, the pilot would obtain server burns or just vaporize altogether due to the frictional heat. (The heat caused by a body moving through air which causes friction.) Not to mention the G’s at acceleration would simply crush or severely injure a human.

Pressurization has nothing to do with anything here, especially with protection from G forces. A cabin can be temperature controlled without being pressurized - consider the interior of your car. You'd need a suit & helmet due to high altitudes, but with or without pressurization you'd hear the same things in the cockpit, just muffled by the helmet a little. The air inside the cockpit, pressurized or not, is still with respect to the pilot, so sound within that airmass propagates normally.

The question being asked has nothing to do with sound inside the cockpit. The question is asking if the sound of the plane can get left further and further behind after the sound barrier has been broken.

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

Sound does not travel in a straight line.  It radiates evenly in all directions.  Therefore, even if your moving super fast, and I mean fast, you would not be able to catch it. I know you're trying to think if you made a really quick lap around the planet could you catch it, you would simply blow through the place it was made. But there are echoes that you could hit if you moved fast enough but even then it would not be the sound itself but a vibration left on the surrounding matter.

Welcome to aviation.SE! Is "a lap around the plant" meant to be "around the plane"? And what does a "vibration left on the surrounding matter" mean?

@Pondlife Most likely, a lap around the planet.

@DavidRicherby You're right, that makes a lot more sense!

Isn't all sound "vibration of the surrounding matter?" Isn't that basically the definition of sound? It's merely a mechanical vibration, no?

Shame the downvoter piranhas got to this answer, it is actually correct.  You could catch your own sound by lapping the planet, but it is easier to slow down and let the sound catch you.  This phenomena is more easily seen on power boats, where the separated high speed wake will slurp back to the transom when the boat slows.

downvotes are because of "even if your moving super fast, and I mean fast, you would not be able to catch it" which is very incorrect.

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Cessna 172 Nose wheel steering
The POH states that ground steering is done by a spring loaded bungee system. How exactly does depressing the rudders apply pressure to the system to actually turn the wheel? Thanks

Although for a different model of Cessna, this image probably illustrates it as well as can be. There may be some differences with a C172 as described below.

Essentially, rods (4) are mechanically connected to the rudder pedals. These move a crank (3), which in turn operate a spring loaded arm and damper combined (2) which moves the nose wheel.
When stationary the pedals can move, and operate the rudder without the need to overcome resistance and move the wheel. However, during forward movement on the ground, the resistance is reduced allowing the nose wheel to turn.
The C172 differs slightly, the rods are spring loaded instead.

The method of operation is also described in the maintenance manual, which you can find examples of online:

The steerable nose wheel mounted on an air-oil shock strut comprises the nose gear. The shock strut is secured to forgings riveted and/or bolted to the firewall and lower fuselage structure. Nose wheel steering is afforded by two spring-loaded push-pull rods from the rudder pedal bars. A hydraulic fluid-filled shimmy dampener is provided to minimize nose wheel shimmy.

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text:
Can I get an EASA Class 2 medical if I have monocular vision?
I have monocular vision (amblyopia) and I want to get the EASA Class 2 medical certificate to do the private pilot license. Will I be able to pass the exam?

As a side note, Carlos Dardano the captain of TACA 110 had monocular vision. This was the 737 that landed on the levee in New Orleans when both engines flamed out during a thunderstorm in 1988. Dardono lost the use of his left eye three years earlier when a bullet went into the cockpit of the cargo DC-3 that he was flying during the Civil War in El Salvador. Carlos  Dardano retired earlier this month after 49 years in commercial aviation, and 38 years after losing vision in one eye.

Yes, possibly. As always with anything medical the key is to speak to an AME. You don't necessarily have to have a full on medical - many will give you some advice/info before you book it.
The relevant info you probably want is:

In an applicant with amblyopia, the visual acuity of the amblyopic eye should be 6/18
(0,3) or better. The applicant may be assessed as fit, provided the visual acuity in the
other eye is 6/6 (1,0) or better, with or without correction, and no significant pathology
can be demonstrated.
source: https://www.easa.europa.eu/en/downloads/1535/en

If it gives you any confidence I have Amblyopia too, with my vision also corrected using glasses, and I have never had a problem getting a class 2 medical. Over the years I've had an faa, an easa and a uk caa class 2.

Got my medical just yesterday!

Medical regulations are complicated, but based solely on the information from your question, it should be possible to obtain EASA Class 2 (but not Class 1).
EASA rules, section "AMC2 MED.B.070 (c)" allow monocular applicants, subject to a few conditions:

Reduced vision in one eye or monocularity: Applicants with reduced
vision or loss of vision in one eye may be assessed as fit if:
(1) the binocular visual field or, in the case of monocularity, the
monocular visual field is acceptable;
(2) in the case of monocularity, a period of adaptation time has
passed from the known point of visual loss, during which the applicant
should be assessed as unfit;
(3) the unaffected eye achieves distant visual acuity of 6/6 (1,0),
corrected or uncorrected;
(4) the unaffected eye achieves intermediate visual acuity of N14 or
equivalent and N5 or equivalent for near (Refer to GM1 MED.B.070);
(5) there is no significant ocular pathology in the unaffected eye;
and
(6) a medical flight test is satisfactory.

Based on my own experience (one eye amblyopic, visual acuity as low as 0,1), they asked to undergo an extensive eye examination with an ophthalmologist and a medical flight test (my AME referred me to both). It was successful at the end, but took a lot of time.


text:
What is the name of the area, controls and instruments where a loadmaster sits?
In a HLA (such as an  Antonov, C5M, or C17 Globemaster III), what is the name of the area where the loadmaster sits?
Also, what are the names of the controls and instruments that the loadmaster uses in these types of aircraft?

I've reworded your question to be a little more gramatically correct, hopefully I have not changed the meaning. Could you clarify what the abbreviation HLA means - I assume something like Heavy Lift Aircraft but couldn't be sure.

thank you for correcting it and yes, HLA I mean stands for (Heavy Lift Aircraft)

In the C-17:
It called the loadmasters station:

Master Sgt. Skip Hurley, 62nd Operations Group standards and evaluations superintendent, briefs Master Chief Petty Officer Mark Rudes, command master chief of U.S. Pacific Command, Oct. 29, 2012, about the loadmaster's station aboard a C-17 Globemaster III aircraft

https://www.airforcemedicine.af.mil/News/Photos/igphoto/2000100799/

https://www.dvidshub.net/image/7239666/89-loadmaster-path-become-c-17-pilot


text:
Do I Set the Parking Brake On the Ramp?
Next week I am flying to an airport and going to have dinner. But I've never stepped out of my aircraft except at my home airport!
When I park on their ramp, do I keep the parking brake on?

do you want a runaway plane?

At my home airport, it's forbidden to set the parking brake. But what if they try to move it while the brake is on?

call ahead and find out. Or ask the tower (if there is one) after you land. If there isn't then check with the guys on the ground.

Great idea! Thanks. I figure if there are tie downs, you likely don't need the brake; otherwise yes?

get yourself a set of chocks and use those

I assume you're talking about a light GA airplane (Cessna/Piper singles or twins).
The short answer is "Maybe, but you probably shouldn't.": The parking brake should only be used for as long as necessary to get wheel chocks in, and should then be released.

Aside from making it inconvenient for the local ramp personnel if they need to move/tow your plane, setting the parking brake has a few other downsides which are related to the way the parking brakes work on your typical light aircraft.
When you set the parking brake you're doing two things:

You are pressurizing the hydraulic lines that run to the plane's brakes.
You are doing something to prevent that pressure from bleeding back out.
(Exactly how you accomplish this varies depending on the plane - the "parking brake" on a Piper Cherokee is a tab that prevents the brake handle from returning to the "released" position, maintaining pressure in the cylinder. I think the Cessna 150 pressurizes and then turns a valve.)

Because of how the parking brake works it has two common failure modes when it's set: Pressure can bleed out or it can blow out.
Bleeding Out happens because fittings, seals, hoses, etc. in the brake system aren't perfect. Hydraulic fluid will escape past whatever little opening it can find, and eventually there will be insufficient pressure in the system to keep the aircraft from moving.
If you happened to be parked on an incline steep enough to get the plane rolling it will then go off on its own down the hill just as if you hadn't set the brake (and you will be very confused when you get back to where you left it).
Blowing Out typically happens on hot summer days when there aren't many little openings for pressure to leak out through: 5606 Hydraulic Oil (the stuff in the brake lines) expands as it gets warm, and with the parking brake set it doesn't have anywhere to go (it can't get back to the main brake reservoir). If you land at 11:00 (local time), set the parking brake, and go have a nice leisurely lunch there is a good chance that when you return to the plane at 13:00 or 14:00 you will find that the hydraulic fluid in your brake system has blown out of whatever the weakest point in the system is (often the seals on the toe brake cylinders, sometimes around the calipers, occasionally a hose fitting fails).
This means you're going to have a mess of hydraulic fluid to clean up at best, and at worst if you're on a hill you're in the same situation as above (the plane has gone rolling away), only now there's hydraulic fluid all over as well (sometimes in a little red trail to wherever the plane has wound up).

So bottom line: Try to avoid leaving the parking brake set for extended periods of time, especially if there might be a temperature increase while it's set.
Instead bring your chocks with you (if you own your own plane, or even if you're a renter, pick up a set of "travel chocks" - they're right at or just under 1 pound, you won't even notice them) and chock your wheels when you leave the plane. Ramp personnel will be able to spot the chock easily, and they're unlikely to fail.
Ideally you would also tie the aircraft down if possible, but some ramps don't have tiedowns (and others are bring-your-own-rope).

Man! So there's no way around asking. I should pick up my own chocks though...Great info, thanks.

That's very interesting about the hydraulic issues, I never thought about that. But in the end the brakes are there for a reason so I guess it comes down to whether - for your particular aircraft - the risk of a failure is important compared to the risk of the aircraft moving. The C172S POH, for example, says to keep them set whenever the aircraft is parked.

A lot of airports have chocks available, but I've been to a good number that don't (or they have to go hunt for them) so I just throw a set in the plane if I know I'll be on the ground longer than it takes to dump fuel inthe tanks & leave. There are cheaper chocks (or you can even make your own), but I like the Travel Chocks: they're study enough that I use them as my primary tiedown chock.

@Pondlife Interestingly I've never seen a Cessna blow out its brake system - I've seen Warriors sitting in a puddle of hydraulic fluid because the fittings at the wheel gave out when it got warm though. (Piper is mute on the subject, at least in my manual: They tell you there's a parking brake, but nothing about when to use it.)

Honestly, 'parking brake' is a total misnomer for these. 'Single point brake application lever' is more accurate.

Why use a hydraulic parking brake, rather than a cable system like road vehicles (including those much much heavier than a C-172) use?

The only (?) downside to setting the parking brake on is that no one can move the aircraft around if they have to (unless you leave it unlocked too). So if you're leaving it somewhere where other people may need to move it without you, you should leave it off.
Lots of FBOs will move your aircraft without you for good reasons: to make room for other aircraft, to fuel it, to put it in a hangar, protect it from bad weather, move it out of strong wind and so on. You can just check with the FBO staff what they prefer, assuming there is an FBO there, of course.
If there's no FBO then the best option is probably to tie down the aircraft in case the wind picks up, with the parking brake on. If there are no tie downs then you really have to leave the brake on because otherwise there's nothing at all stopping the aircraft from moving. And if you regularly fly places with no FBOs and no tie downs then you should probably consider bringing your own.


text:
What does this star symbol mean in the airport chart?
I know that there is no unidentified beacon because it is not in the circle, because the star symbol inside the circle represents the unidentified beacon. So what does this star symbol mean?

According to the FAA Airport diagram legend both a 5 point star with and without an enclosing circle both identify a beacon at the airport.
Further details can be found on the FAA website, towards the bottom of this page is a link to Legends & General Information (PDF) where you can find all details of all FAA charts.

You are right, sir. But when I check out the Jeppesen Legends I do not find any other information than the hazard beacon. It is not possible to have such a thing on the terminal.

it sure can be. Maybe there's a tall mast or flag pole on the roof with a rotating beacon light on top for example. Think that kind of beacon, not radio beacon.


text:
Has there been any attempt to use an underground cable system, a la a cable car, to tow aircraft along taxiways?
I've been looking at some proposals to try to reduce the time that an airliner's main engines are run at idle or near idle when taxiing to and from runways--a situation that uses non-trivial amounts of fuel and greatly increases local emissions.
I've seen proposals for fleets of tugs and onboard wheel motors, but I wonder if there might not be a simpler way. Why not use a continuously moving cable system, such as that used by San Francisco cable cars? Simply run a cable in a trench in the center of a taxiway, and have a means of attaching to and detaching from this cable as a way to tow aircraft down a taxiway and to the end of the runway. I don't know the details, but I do know cable car designers figured it out more than a century ago, so surely it can be done.
Similarly, one might also utilize a roundhouse system to rotate aircraft to the proper position to pick up the cable. Again, roundhouses have been used with trains for well over a hundred years, and locomotives are quite heavy. Clearly one would need ways to slow the cable speed at termini, but this is done on high-speed ski lifts. Some means of "clutching" to buffer a sudden application of force would also be in order--but again, think of the cable cars, or steam catapults/deceleration cables on aircraft carriers. It's been done. Surely it can be done in this context.
What's not to like? The benefits of reduced fuel consumption and emissions are obvious, and it beats a flotilla of tugs, by eliminating empty return trips, logistical hassle, and large numbers of staff. Also likely beats an onboard motor system, on weight, cost, and complexity. There are tons of reasons to do it.
Seems obvious to me. What are the arguments against it?

Not for taxiing, but there are similar systems in place for launching planes from aircraft carriers.  These are obviously much lighter aircraft, not jetliners, and it's not a continuously moving cable so much as a steam (or in one case electromagnetically) driven shuttle plate embedded in the deck of the carrier.  But it is planes being dragged by a hook underneath, in this case fast enough to launch off a short deck.

In addition to what Darrel mentioned about aircraft carrier fixed-wing aircraft, recreational and commercial gliders as well as recreational and commercial drones are sometimes launched via a cable system.

related: https://aviation.stackexchange.com/q/24479/3159

If a tug breaks, another will take its place. What happens if your system breaks? Will all planes ground to a halt? The current system is very simple and flexible. Not to mention once you've committed to a system, it's very expensive to replace.

Related: https://aviation.stackexchange.com/q/9101/1626

I was in a museum in the Netherlands in 2017 and saw an exhibition called I think "100 years of Schiphol" on the KLM airport. One of the exhibits was about sustainability efforts and included a video about a hypothetical system for landing planes which did not involve wheels. Instead, as a plane neared the runway, its speed was matched by a sort of "sled" on the ground running on a kind of rail. The video showed the plane somehow docking to this sled to complete its landing. I did some web searches, but unfortunately could not this video or any mention of such a system online.

@Michael That was luchtvaartmuseum Aviodrome with 100 jaar Schiphol. I don't think the media you're looking for are on-line.

Cable cars are some of the least energy efficient forms of transportation. Less than 2% of the energy used to power the system propels the vehicle. The rest is lost to moving the cable, turning the pulleys.

As many answers point out, one large problem is the cable itself, and how much of it you'd need to pull around to let different planes move in different ways as needed.  (And how big/heavy it would have to be).
A real proposal is a similar idea, but with electric power instead of cable towing.  https://news.okstate.edu/articles/engineering-architecture-technology/2020/osus_new_product_development_centers_collaboration_with_aircraft_towing_systems_leads_to_prototype_test_launch_in_spring.html  (@mitchute posted this link as a link-only answer).

ATS’ innovative aircraft transport system includes an electric-powered underground railway system.

“With a fully installed ATS system, a pilot can land on a runway and move onto the taxiway, where he or she can align the aircraft’s nose wheel into an ATS tow dolly,” Taylor said. “Once the nose wheel is secured, a pilot can turn off the main engines and ATS safely moves the aircraft using its channel system to the appropriate gate. The reverse process is used when pilots are ready to take off.
“Aircraft are simply automatically transported from the gate to the taxiway and then the runway. Pilots then turn on their engines and drive off the pull car, move into position and take off.”

So more like a subway (where trains draw electrical power from a 3rd rail), I think: there are fixed routes, but they're just rails with a mechanism for a powered "tow dolly" to get electrical power for itself and use it on the spot.  So it can easily use sustainably-generated electrical power without needing onboard storage for it.
The tow dolly itself might have normal wheels, and only need the rails for power, and/or traction to pull a big plane, IDK.
Any specialized equipment for coupling / decoupling from different rails at intersections can go in this tow dolly, not on the plane itself.  (Although you could do the same thing with tow dollies pulled by a cable, with a grabber motor powered by onboard batteries?  Since you probably want some smarts and other motors in your tow dolly, electricity is an advantage over purely mechanical pulling)

While it might be feasible to build a tow-cable system, there are clearly better alternatives that would give a very similar result.  i.e. I don't think there are many disadvantages to electric tow dollies that you wouldn't also have with cable towing.  So there are probably no cases where cable tow would actually be the best choice.
Tow-cable systems waste a lot of power on cable friction.  Building efficient high-power electric motors is a solved problem.
Other answers and comments have pointed out large potential problems, especially turbine cooldown time: you have to leave them on for some time anyway, or somehow have them spinning to move air.  But not all even have an electric motor that could do that if supplied with power.

Wow--I LOVE OSU's solution! I've actually been inventing up a storm in my little pea brain since I asked this question, and I came up with something very similar--electrified rails, towbarless tug/locomotive, turntables. I found a neat conceptual video they came up with a few years ago, on YouTube: https://www.youtube.com/watch?v=AOu-vBZWqHw. It's a very exciting concept, and I would love to see these guys succeed.Rail is almost certainly the way to go, for traction, low rolling resistance, weight accommodation, staying, well, on track, durability.

Way to go on thinking outside of the box. Actually, it might work. Although the logistics of it are above my pay-grade. Maybe, some smart group out there could make it work.
But, your question doesn’t ask for ways to make it works. It specifically asked for reasons it might not works. So, here it goes:

cable car feasibility over all does not have a great track record. If we were to make a Ben Franklin style decision graph of cable car systems that worked versus those that proved ineffective, Wikipedia lists a lot more defunct systems than it does working systems. All of which run on tracks.
If you get more than two runways at the same airport, the taxiway system become increasingly complex. An aircraft would need an alternate means of locomotion when switching from one runway/taxiway to another. The use of one continuous loop cable would be rendered impossible.
As Ron Beyer has pointed out, departing aircraft would still need time to warm up their engines prior to take off. Currently, that time is spent taxiing. Arriving aircraft would still need to power climate control systems. It would be a cheaper and more efficient use of time and resources to design a system that eliminates wait time between start up and takeoff. Keeping the traffic flow moving at all times without having to retrofit airports or entire airline fleets would benefit all.
Most cable car systems operate with cable cars of lighter weight and similar mass to each other compared to commercial aircraft. How would airports be able to account for the sheer mass differences between a single pilot C208 after a long cross country versus an A380 taking off at MTOW.
Power failure.
Power in night time IMC with four or five airliners about to break out at minimums for GPS approaches on parallel runways.
In the case of a constantly moving cable, an aircraft the size of an A320 could cause a lot of damage if it were not able to unhook itself from the cable at the appropriate time,
According to Wikipedia, the efficiency of a large cable car system is questionable. “Approximately 95% of the tractive effort in the San Francisco system is expended in simply moving the four cables at 9.5 miles per hour.“

Thanks for the good input! OK, so I am definitely liking the scheme a lot less now. I am not a pilot, but it seems there is some thought that maybe 5 min of warmup is sufficient, rather than 25-40. If so, what about this? https://www.taxibot-international.com/ . Looks like it's been built and is in limited use. Doesn't the APU provide sufficient power for climate control, etc., such that main engines wouldn't be needed?

@lapsedwondering - I believe the APU does provide enough power for climate control. I am not sure what the cost or pollutions savings would be on running an APU vs an engine at idle. I would also check with someone more knowledgeable on engine warm up and cool down time on an airliner. Five minutes would be the absolute barest of minimums I would try for a piston driven GA aircraft. Generally, start-up, clearance delivery, avionics programming, taxi, run-up, and time at the runway hold short line totals about 15 minutes. 10 minutes at the quickest.

The minimum warm up time for a big jet engine is more like 20 minutes, There is also a minimum cool-down time running at idle. If you just stop a hot engine, the heat rises and thermal expansion bends the rotors because the top of the compressors and turbines are hotter than the bottom. That can be bad enough to make it impossible to restart the engine without exceeding the vibration limits. Some jet engines have small electric motors which keep the rotors turning slowly to avoid this, but it's an added complication and another potential failure condition.

+1  Also, taking control of taxiing away from the pilots would mean they couldn't stop short as happened in this incident: http://www.avherald.com/h?article=4d8b926a&opt=0

Also, what happens when it snows?  The cable has to run in a trench, which has to be narrow enough so that tires will not fall into it.  Get a good snowstorm, and it fills up with snow, which likely turns to ice.  At best it greatly increases the energy needed to move the cable; at worse it locks up the system and you have a heck of a job cleaning it out.  (Note that per the Wikipedia link, the only still operating cable car system is in San Francisco, where it basically doesn't snow.  It's also primarily a tourist attraction...)

@jamesqf - You are entirely right about the snow. Although, the San Fran system is not the last operating cable car. It is the last one that is operated manually for nostalgic purposes for tourist. I think San Fran also had a unique situation with the hilly terrain back when horsepower was limited to as little as one

@jamesqf if we're retrofitting planes with hook spots, we might as well retrofit them to have four wheel assemblies instead of three. Although I do admit it adds weight...

@John Dvorak: We don't have to think about just the commercial airplanes, but about all the maintenance vehicles running around.  Not to mention all the small GA planes that want to use the airport, too.  And military: quite a number of fighter jets use my local commercial field.

"cable car feasibility over all does not have a great track record." --> I suppose no pun intended?

@DeanF. the modern age hasn’t provided anything that can handle those hills.  Adhesion-rail streetcars are right out.  Trolley buses can’t quite climb the grades either, although maybe they now can with flux vector AC drives.  They’ve had Twin Buses, diesels with 2 engines.  It’s not an easy problem.

@chux-ReinstateMonica - maybe.

That #8 is what kills it.  A tug car would be at least 30-50% efficient and wouldn't be limited to pre-set routes either.

@Harper - Reinstate Monica: Actually there are a number of systems that can handle steeper grades.  Per Google, the steepest grade in the SF cable car system is a mere 21%.  Cog railway systems (common in Switzerland) regularly handle more than that.  The steepest does a 45% grade: https://en.wikipedia.org/wiki/Pilatus_Railway

Practically this would not work due to the sheer size of the cable you would need. A fully loaded A380 is 1.25 million pounds. A B787 is .5 million pounds and a B737 is about 150,000 pounds. On a busy day at a large airport you may have 20 airplanes or more in line, assuming a mix of airplanes you'd have somewhere around 5 million pounds of airplanes to tow around. I don't have the figures, but the cable would have to be very thick to handle that load. Thick cables are heavy to begin with, and you'd have miles and miles of it to move, which takes a lot of energy. Curving large steel cables has to be done gradually, and needs substantial mechanisms, all of which would have to be maintained.
It would be expensive to build and maintain, and likely cost more energy to run than it would save.

“More energy to run than would save” that’s highly unlikely.  The APU on an A320 could run San Francisco’s entire cable car system along with OAK’s cable car too.

Figures: 5 million pounds to tow around with a rolling resistance of 0.015 would require around 330 kN. That's almost in the range of a 52 mm rope. Not completely out of the realm of possible solutions.

So about 2 inches @Arsenal?  You'd have to over-spec it somewhat I would assume, in any case that's a chunky old cable!

@GdD I guess the hooking and unhooking would put quite a bit of extra stress to it, as that won't go very smoothly - not quite sure how that would be solved at all.

I'm not sure how that would work, frankly @Arsenal. There would have to be a way to put airplanes on it and take them off without stopping or nobody would get anywhere. They have mechanisms for chairlifts I suppose, so something like that I would guess. Having to pull a million pound airplane to taxi speed is no small force.

@Harper-ReinstateMonica Neither of those cable car systems haul 20 airliners at once

@AsteroidsWithWings Dude, they’re trains. They are a type of “heavy” aviators do not grasp. What’s more, they’re going up a frickin’ 60% grade, at max speed, because there’s only one speed lol.  Tow force of an A380 on the flat vs a fully loaded Powell-Mason car at ruling grade... tough call.  Good one for Mythbusters!   It helps that while some cars are going up, others are going down.

@Arsenal The size of the rope alone is definitely not the problem, I've handled electrical cables thicker than that.

I'm not saying it can't be done @Harper-ReinstateMonica, simply expensive and impractical for the purpose.

@GdD “Yeah, near every city, we’ll build an ultra-high-security area, that is itself the size of a city. This will house 2-1/2 mile long, 200’ wide strips of the thickest concrete ever laid. Machines the weight of a Big Boy loco (with turbines that burn fuel inside the turbine) will use these as a dragstrip to get to 150 mph in half their length, leap into the air itself, and travel halfway around the world with onboard fuel only, in, the, air, in hours. We will tow these leviathans to the end of the dragstrip with a cable system.”  Hmm, that cable system sounds very impractical.

I detect sarcasm captain!

Cable cars, trains, and ski lifts essentially live their lives on the track or line. The repositioning and shuffling that must happen on an airport, especially pulling into and out of arrival/departure gates would make such limited track systems a nightmare. There would be a need for aircraft to move around under their own power or be towed by a tug, at least in non-movement areas, and if they already have their engines started or have a tow vehicle, it makes no sense to complicate the process by adding a track for only part of the journey. Additionally, there are plenty of airports where taxiing traffic may need to go both directions on a given strip of tarmac, yielding to traffic as needed. You would need 2 cable tracks going in opposite directions and some way to connect your aircraft to the right one.
There are certain select aviation applications for a cable tow. For example, certain airports with a lot of glider activity have a glider winch which pulls the unpowered aircraft down the runway and releases the cable upon takeoff. Nevertheless, underground cable tracks on a large, busy airport would be too limiting to be worth the monetary investment and time spent in procedural development.

Hmmm. Well, the grabbing and ungrabbing seems to have been worked out: https://www.streetcar.org/wheels-motion/cable-cars-work/#:~:text=Cable%20Cars%20have%20no%20engine%20or%20motor%20on%20the%20cars%20themselves.&text=There%2C%20powerful%20electric%20motors%20(originally,that%20slot%20between%20the%20tracks).

I’m not saying the grabbing and ungrabbing would be a problem. I’m saying, street cars have side-by-side tracks for going in opposite directions. Sometimes a single taxiway will need to accommodate centerline traffic in either direction. Don’t get me wrong, I think your idea is very clever. You just asked for arguments against it, so I gave it some thought. I’m glad you joined the site. Hope to hear more from you in the future.

No worries at all--I was asking for arguments against, and thank you for the thoughtful response. Mostly throwing out an idea, not at all identifying with it. I do see the problems posed by engaging and disengaging a continuously running cable, and then there are further challenges of dismissing whatever hook or boom system one might not want to take along on a flight. And yes, huge initial costs and logistics problems.

@AaronHolmes that’s not a problem, just have 2 cables to grab.   I believe a section of SF’s cable car does this.  Or you could just reverse the cable drive; that’s what OAK’s cable car does.

Look closely at the landing decks of the more modern Aegis / Arleigh Burke destroyers.  There is a slotted trackway from the landing deck leading into each of the two helicopter hangars.

Note, too, that the deck of a destroyer has significantly fewer source/destination combinations for an aircraft to travel between and significantly fewer aircraft types to deal with, making this more practical.


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When are aircraft required to dump fuel for emergency landings?
I've heard that in a lot of instances a jet that is making an emergency landing is required to dump excess fuel.  In a lot of instances this would make a lot of sense.  For example, if the gear cannot be lowered and the plane must land on its belly.   The last thing you want is a hundred tons of jet fuel involved.
But, I have to assume there are also emergencies where a plane isn't required to dump its fuel.  I just am not sure what they would be?
So, my question is: When are aircraft required to dump fuel before landing (or, at very least, when is it advisable to dump fuel before landing).
Bonus points:

How much fuel does an aircraft retain after an emergency dump? I'm
assuming they don't dump it all or they would have no fuel to
maneuver with.
Examples would be great.

My understanding is the major consideration is usually weight - large aircraft typically take off at a weight higher than their maximum landing weight (with the intent to burn off the excess weight in fuel during transit). If an emergency arises they may not have burned off enough fuel, and will have to dump the excess.

For many medium and large sized jets the maximum gross takeoff weight is higher than the maximum landing weight.  If the airplane has an emergency that requires an air return or other landing in the early part of flight, it is very likely overweight for landing.  The plane has 3 options at this point:

Land overweight
Dump fuel (if able)
Fly around at low altitude and high power to burn fuel

For some airplanes, option 2 is not available.  This includes airplanes that are not equipped to dump fuel (not all airplanes have this capability or have the capability under MEL) or are not approved to dump fuel at their current location and altitude.  For these airplanes, only option 1 and 3 are available.
The choice between option 1 and 2/3 comes down to what kind of emergency it is.  If you have to be on the ground now, you won't care about being overweight, and will just put it down.  If you have time and landing now isn't a priority, then you would choose to dump or burn fuel.
If an airplane chooses to dump, they will generally choose one of two values of remaining fuel:

fuel such that they land at the maximum landing weight
absolute minimum safe fuel to land

This choice comes down to how likely the landing will end in an accident.  If you are sure the landing will be a non-event (e.g. air return for anti-ice failure) then you'll likely just dump until landing wont be overweight.  Conversely, if you have a control issue or something that may end badly, you want minimum fuel to lessen the potential for fire.  In these cases the ARFF crew will spray the runway with foam to assist in reducing the fire potential.

So it's all just up to the pilot?  Or are there cases where there are guidelines that must be followed even if the immediate usefulness is not apparent?

The pilot is the final authority on all aspects of operating the airplane, including the decision to dump fuel.  The suggestion to dump fuel for a specific scenario is probably suggested in the QRH for whatever problem the airplane is experiencing.

It is no longer advisable to  foam the runway in fear of running out of foam if the actual fire occur

@vasin1987: And because it increases the chance of the aircraft sliding off the end or side of the runway and thereafter catching fire.

The primary reason is to reduce the weight of an aircraft. Aircraft are designed to land at a maximum weight lower than their maximum weight. In order to be under that maximum landing weight, they must either burn that fuel during flight or dump it.
There is no requirement for them to dump fuel. This decision is up to the pilot. They may choose to land the aircraft "overweight", meaning over the maximum landing weight. In this case the aircraft will have to undergo an inspection to ensure that nothing was damaged. On some aircraft, an inspection is only required if the touchdown acceleration was over a certain amount (like 1.7G).
Dumping fuel will generally be an issue with a plane that has taken off for a long flight and has to divert for some issue. The pilot must decide how urgent the emergency is. If it is an urgent problem, such as engine failure, medical emergency, hydraulic issues, or fire, their priority is to land ASAP regardless of weight. They may dump fuel while headed to a diversion airport if they can.
If the emergency is not as urgent, the pilot may decide that they do not want to continue the flight, especially if it involves long stretches over water without diversion airports. In this case they can hold somewhere near a diversion airport to burn off or dump fuel until they are comfortable with their landing weight.
For example, the 777-200ER can take off at 656,000 lb, but maximum landing weight is 470,000 lb. This means they could have to dump 150,000 lb of fuel if they encounter an issue at the beginning of a long flight.
The amount they leave after dumping will be up to the pilot. For serious gear issues, they may want to be as light as possible and only leave a minimum amount of fuel in the tanks (on a 737, this is about 4,000 lb). For other instances, they may be satisfied with the maximum landing weight and not want to spend any more time dumping or burning fuel. No pilot wants to dump a bunch of fuel and then end up needing it later.

(+1) Note that "an issue" can be an external one, e.g. the closing of US air space on 9/11. (On an AMS-BOS flight soon after take-off.)

Regarding that last paragraph, the question of how little fuel you want to have left on a 737 would be mostly academic unless you're fine holding for hours - 737s can't dump fuel.

In reference to how much fuel is retained, in the 747-100 and -200 aircraft it was not possible to run yourself out of fuel by dumping. There were stand pipes in the the system that prevented that. A common call to the flight engineer in the simulator for certain emergencies was to "dump down to the stand pipes." Offhand I don't remember how much fuel was left when that was done.
The most fuel I ever dumped to get down to max landing weight (635,000 lbs as I remember) was around 100,000 lbs. We did it at 4,000 feet over the ocean off Lima, Peru after an engine blew (it literally disintegrated inside) on takeoff. As I remember, we believed that dumped fuel evaporates in 2 to 3 thousand feet of fall, so we did not believe it would reach the water. Not that that was any consideration at the moment.

I was wondering if fuel did that (evaporate) in its fall to earth. I guess doing it over a non-populated area is just for safety precaution then.

@JeffBridgman My guess is that the 2 to 3 thousand feet is a general figure. At Lima, it's hot and dry. If it were cold and damp, I would expect it to take longer; how much I do not know.

@JeffBridgman Here's a few questions I found (all of them more recent than your comment) which may be relevant: How is fuel dumping safe? and Is it possible to recover dumped fuel? and Why isn't dumped fuel burned? and What is the minimum altitude for dumping fuel?

The reason they dump fuel is to keep the landing gear intact during the landing. A plane at max takeoff weight will not be kind to its gear. Max takeoff weight has to be equal or higher is also higher than max landing weight. A plane where max takeoff weight is within 5% of max landing weight doesn't need to be able to dump fuel.
When there isn't a designated fuel dumping site (typically a nearby lake or ocean) or the plane can't make it there to dump fuel, they will instead circle around until the fuel level has dropped sufficiently. This is not possible when the emergency is of the kind that needs to be on the ground NOW.
A recent incident that had a fuel dump is Air France B773. Another incident where they couldn't dump fuel is Delta B763.

Thanks for the examples @ratchetfreak, always very instructive.

10 or 20 years ago there was a plane taking off out of LGA or JFK that immediately had an emergency and dumped a lot of fuel over Queens or Brooklyn. No fire on the ground, but some homes were literally soaked and suffered damage to asphalt roofs (I would imagine street and driveway damage too). The plane landed safely.

SwissAir 111 was circling near Halifax when it went down. Was this to burn off fuel? If so, were they unable to dump for some reason?

@PhilPerry: SR 111 went there to dump fuel, and did start doing so a handful of minutes before it crashed. Dumping is quicker than burning it but not instantaneous.

@PhilPerry Yes Swiss Air 111 was dumping fuel when it crashed. With hindsight, that was one of the cases where they should have just got the plane onto the ground ASAP, even if it meant landing overweight.

@DavidRicherby well hindsight and all that

@ratchetfreak Yeah. If they'd had the benefit of hindsight, they'd have just stayed on the ground in the first place.

@DavidRicherby: According to the official accident report, Swissair 111 had been dumping fuel before it crashed, but was not actually dumping at the time of the crash.

Having spent 3 days with one of the investigators of the Swiss Air 111 flight during an international safety seminar, he said the dump was unnecessary as that bird was certified to land at its current weight when an emergency was declared. That very basic mistake cost all lives, as had he flown direct to land, he would have been on the ground before the fire was big enough to impact the cockpit. Both pilots had donned O2 masks. The fire burned through the copilots oxygen hose and turned it into a blow torch to the side of his head before they hit the water. This bird hit the water at an angle and speed which caused extreme water pressure to be forced into the fuselage. Pressures were so great, the water entered passengers bodies orifices and many had the top of their heads explode. He said it was a gruesome job for the recovery workers.

This is an interesting insight, but it doesn't really answer the question of when aircraft should dump fuel

A very basic answer to when an aircraft should dump fuel is when the checklist calls for it. Otherwise, in any aircraft I've flown that had such a capability, fuel dumping was only advised as a contingency to prevent landing overweight during an abnormal operation. Many aircraft have a much higher takeoff weight than they do landing weight. My current aircraft when topped off may fly up to 2.9 hours before reaching landing weight. I do not have the capability to dump fuel, however. Should I be forced to land before I reach my landing weight, I am required to have the aircraft inspected prior to the next flight.


text:
How is fuel dumped in aircraft?
I have heard incidents in which commercial aircraft are required to dump fuel in event of an emergency to avoid landing over maximum landing weight. What systems do aircraft have in place to dump fuel? How is a fuel dump initiated from the cockpit?

related: http://aviation.stackexchange.com/questions/2854/when-are-aircraft-required-to-dump-fuel-for-emergency-landings?rq=1

@ratchetfreak that question prompted me to ask this :)

@mins your answer is good and I upvoted it.

Commercial aircraft have fuel dump nozzles located under the wings for dumping fuel. Fuel is gravity jettisoned from the tanks located in the wings and the fuselage. Large commercial aircraft such as the 747 can dump upto 6000 lbs of fuel per minute with all fuel jettison pumps open.

Interestingly, the fuel dump nozzles are in different places on different aircraft:
On the 747, the nozzles seem to be located near the wingtips:

Whereas on the A380, the nozzles seem to be much closer to the fuselage:

It is triggered by activating the FUEL JETTISON switches in the cockpit on the overhead panel. Below is a picture of the same in an Airbus A380-800. Both the ARM and ACTIVE switches must be pressed to start a fuel dump.

I always wondered what those nozzles were. Never thought to ask. Very interesting!

Now I'm wondering why there are pumps. That must add weight, and there's already plenty of pressure differential around wings. If the nozzle is positioned in a low pressure area, the fuel would be sucked out.

@MSalters Typically the pumps that pressurize the fuel system are also used to jettison fuel. On the 747-100 and -200 aircraft for example, if you need to jettison fuel in the center tank, the pumps there are referred to as the override/jettison pumps, override because they produce higher pressure than the other pumps in the other tanks, and thus when on the engines will use fuel from the center tank rather than the wing tanks. Also the same plumbing used to jettison fuel is that which is used to load fuel, the difference being the positioning of valves in the system.

@MSalters In an emergency situation where the airplane is not going to be able to continue flying until you reduce its weight, you need to reduce that weight as quickly as possible, and mere gravity and differential pressure around the wings would not give you the flow rate the pumps do. A common call by the Captain in such an emergency in the simulator is to "dump down to the stand pipes", referring to the plumbing that keeps you from accidentally dumping all your fuel.

@MSalters In fact, smaller commercial aircraft such as the 737 and A320 don't even have fuel dump systems in place!

@shortstheory A plane only needs to be able to dump fuel if max takoff weight is more than 5% higher than max landing weight.

You may download the fuel system description for a B747-400 here.
(wait for the 20s countdown to complete).
This is a part of the "747 Flight Crew Operations Manual".

(Note that all pictures are extracted from the Boeing Company documentation, available on several web sites. Some copyright applies, see Smart Cockpit which provide this copy. Also note that this documentation is definitely outdated, and is not intended to be used for actual operations)

Here are the basic information for the fuel jettison operations.
The overhead panel (which is above the head of the crew) has some controls:

1: Allows to select the quantity of fuel to remain in the tank (jettison is used primarily if the aircraft must be landed shortly after takeoff, not much fuel has been burnt, and the maximum weight allowed (or desired) for landing is not yet attained).
2,3: The button to activate jettison, left and right nozzles, and their lights.
4: The selector to arm the jettison system.
One of the information screen (EICAS) will display the quantity to remain:

The can also sees the status of the fuel tanks on the other EICAS screen:

1: This is a jettison nozzle.
2: The time required for the jettison operation (based on actual fuel flow).
Jettison schematic
(depends on the aircraft model, the documentation shows different models).

You can see pumps and valves (pairs of blank half discs)
Fuel jettison mode of operation

The fuel jettison system allows jettison from all fuel tanks.
  Override/jettison pumps in main tanks 2 and 3 and the center wing tank
  pump fuel overboard through the jettison nozzle valves.
Fuel jettison is initiated by rotating the fuel jettison selector to A
  or B. When a jettison control system is selected, the fuel temperature
  indication on EICAS is replaced with the fuel to remain quantity
  indication. The jettison manifold and jettison time display on the
  fuel synoptic. Rotating the Fuel To Remain selector decreases or
  increases the fuel to remain quantity.
Pushing either fuel jettison nozzle valve switch ON activates all
  override/jettison and transfer/jettison pumps in the tanks containing
  fuel (pump switches must be ON) and opens the required jettison and
  transfer/jettison valves. The related jettison nozzle valve also
  opens. The jettison time is initially estimated using preprogrammed
  rates. The system begins updating the estimate based on actual fuel
  quantity rate of change ninety seconds after jettison begins.
Jettison terminates when total fuel quantity decreases to the fuel to
  remain quantity. The fuel to remain quantity indication changes color
  from magenta to white and flashes for five seconds. The jettison
  control system deactivates all operating override/jettison and
  transfer/jettison pumps. The related FUEL OVRD pump EICAS messages
  display until the Fuel Jettison selector is OFF.

Now see what it looks like from the cabin  , when all this expensive fuel is evaporated in flight (YouTube video).

Now that is interesting, is this the real 747 training manual?

@shortstheory:This is the FCOM (F/C operation manual), it is usually customized by tail number, because each aircraft may have different options chosen by the A/L. The manual you are talking about is usually known as FCTM (training manual). Search FCTM, you will have a lot of results. Here is one for the B737. And yes these are real "not customized" manuals. If you are interested, other documentation is known under names like "SOP" (standard operating procedures), "QRH" quick reference handbook (hot situations)...


text:
Is defacing the airline logo of a wrecked airplane illegal?
I was listening to episode #25 of the Hello Internet podcast where Brady Haran mentioned that it's not unheard of for airline employees to rush to their own airplane's respective accident scene to deface the logos and registration numbers painted on the airplane's wings and fuselage. This is apparently done to reduce the damage to the airline's reputation. Below is an picture of the same from an alleged account of Thai Airways defacing the logo on a crashed (but still very intact) airplane.

So is there any law regarding whether this can be done? Could defacing the logo end up hampering the air crash investigation?

It doesn't look like it's forbidden by the ICAO, so it would depend on jurisdiction. Do you want to limit your question to China?

This is a very common practice. It helps prevent the media from identifying the airplane and making a big deal out of the crash - hurting the companies image. I remember coming to my flight school one day and there was a totaled Cessna 172 on a trailer with the tail number and flight school logo taped over. I never heard a word of it in the news, so this practice definitely works - probably better for flight schools than airlines.

@raptortech97 Not necessarily, but I would be interested to know about whether the jurisdiction differs over there.

Personally, I don't see the point in hiding the logo when you got your company colors sprayed all over the craft, never mind that you can probably just look up all relevant information on the arrivals page for that air port's website.

@NateKerkhofs that would be true for most of the members of Aviation SE but IMO 90% of the public won't bother to check out which airline it is just from the pictures.

@shortstheory I'm mainly talking about journalists, who will share that info with the 90% that doesn't check.

In the US (you tagged your question FAA) it would most likely be illegal. 49 CFR 830.10 says (note that the NTSB investigates US accidents, not the FAA):

(b) Prior to the time the Board or its authorized representative takes custody of aircraft wreckage, mail, or cargo, such wreckage, mail, or cargo may not be disturbed or moved except to the extent necessary:
(1) To remove persons injured or trapped;
(2) To protect the wreckage from further damage; or
(3) To protect the public from injury.

It seems highly unlikely that painting over logos would be justified for any of those reasons. On the other hand, this wording leaves the question open of what might happen after the NTSB takes custody, and in theory an airline could do it with permission.
Personally, I find it hard to see how this would help the airline because the story would be all over the news anyway, but laws and attitudes vary widely in different parts of the world.

After the NTSB takes custody they can give permission, on the basis that the logo (or more importantly, hull under/around it) isn't important to the investigation. The rules are to prevent accidental or deliberate destruction of evidence

This is done in cases where it is not “all over the news”. A runway excursion requiring evacuation with no injuries or maybe one or two sprained ankles will get a mention in local newspaper (and on AvHerald) but that's about it. It is also usually only done when it is on an airport where the travellers can see it.

Also, pictures posted on the internet are forever.  From a PR point of view it makes perfect sense to minimize the possibility of adverse publicity.

@JanHudec I'd buy this as a PR save if it weren't for the fact that the rest of the livery is usually a pretty good indication whose plane it was -- Google Airline livery red purple orange gold and you'll find that the jet above is strikingly similar to a Thai Airways paint scheme. If a news reporter wanted the airline name that's a pretty trivial way to get it. But hey - if it makes the airline feel better!

Well, @voretaq7, I was wondering myself why they only blackened the logo and number as it still made the owner easily identifiable especially given there were more aircraft of the same airline around. In the other cases the aircraft was repainted more thoroughly. Like in this case.

@voretaq7 That's the thinking behind the last sentence in my answer: in some parts of the world most people still get their news from newspapers and internet access is (relatively) limited and/or controlled. In that environment I can imagine a state-funded company (like Thai Airways) locking down the reporting, even if in a more tech-savvy and open society it looks pointless. But I'm not exactly an expert on media trends and press freedom, so I may be way off track on this.

@Pondlife: I don't think it would slow down serious reporters anywhere, but this kind of accident rarely makes front page; more like short column on third page. It's more to limit panic among travellers who see it and perhaps tweet/facebook/googleplus/… it (and never bother to look anything up).

Most people can read (at least in developed countries) and can retain names (e.g. company names), while a smaller fraction will recognise the livery. For example I personally can recall the "Asiana" accident but I don't know how they paint their aircraft.

@voretaq7 However, if I (an amateur) accidentally see the photo above, I'll remember these two things: 1st there was a plane with crashed front wheel, 2nd it was purple and white. OTOH, if I see a large THAI sign over it, I'll much more likely remember the following thing: Thai planes are not safe. This small trace in my brain can influence my decisions, whence their sales.

Would covering the tail by a large white poster be considered as "disturbing"? It's an action that is easily revertible.


text:
Is this an external cannon on an F-16?
What is the black pod starboard of the front landing gear on this F-16-I? At first I thought that it was a laser finder, but upon closer inspection it seems to resemble some type of short cannon. What might it be?

What you seem to have found is the Litening Targeting Pod.

Northrop Grumman's widely fielded LITENING system is a combat proven,
  self-contained, multi-sensor targeting and surveillance system.
  LITENING enables aircrews to detect, acquire, auto-track and identify
  targets at extremely long ranges for weapon delivery or nontraditional
  intelligence, surveillance and reconnaissance missions. Source

pod http://media.defenceindustrydaily.com/images/ELEC_LITENING-AT_on_F-16C_lg.jpg
The F16 does have a cannon as well, but it's located on the port side under and aft of the canopy:

Thank you! Might I ask how you found that? My google-fu must need some improving.

@i googled 'f16 weapons', came up with this image and was able to make out something about a pod that looked similar :) happy to help

Mind that an external gun pod for the F-16 was created when it was planned to replace the A-10 with the F-16. It was the GAU-13 and used a scaled down version of the A-10s GAU-8 gun. It was however never produced in numbers and never entered operational service because of its poor accuracy.

At the start of Desert Storm the USAF did in fact deploy a squadron of 'A-16s' to perform CAS and ground attack using a podded GAU-8.   Unfortunately, the only place on an F-16 (of that era anyway) to load an ECM pod is the centerline station, which is also the only place the GAU-8 can be loaded.
It only took a couple of near miss SAM engagements before the 'A-16' was converted back into a proper F-16.